XRF is a well known technique for measuring properties of samples. A number of different configurations are known.
In wavelength dispersive XRF (WDXRF), X-rays from an X-ray source are incident onto a sample. These cause X-ray fluorescence, i.e. X-rays emitted at a range of energies that act as a signature of the sample material. In a first arrangement, illustrated in FIG. 1, X-rays are emitted by tube 2 as the source and then are incident on sample 4. The X-rays from the sample are incident on a flat analyzer crystal 6 which diffracts them according to the Bragg equation onto a detector 8. By moving the analyzer crystal 6 by angle θ and detector 8 by double the angle 2θ, the wavelength of X-rays diffracted by the flat analyzer crystal into the detector changes and so the movement of the detector allows the measurement of a range of wavelengths and hence energies. The analyzer crystal provides good discrimination between different wavelengths. The X-rays pass through parallel plate collimators 11, 13.
A variant of this arrangement uses a curved analyzer crystal 10 as illustrated in FIG. 2 in combination with a first slit 12 adjacent to the sample 4 and a second slit 14 adjacent to the detector 8. The curved analyzer crystal 10 acts as a monochromator only imaging X-rays at a particular energy passing through first slit 12 onto second slit 14.
In this approach, the curved crystal 10 provides the discrimination between different wavelengths, and hence energies. In general, a different curved analyzer crystal is used for each energy. Alternatively, the curved analyzer crystal 10 can be mounted on a goniometer and rotated in a similar manner to the approach of FIG. 1.
In order to measure at a range of energies an alternative approach known as Energy Dispersive XRF (EDXRF) may be used. In this approach, the X-rays emitted by the sample are measured directly in a detector that can measure the intensity as a function of energy. Such a detector may be, for example, a silicon drift detector that can measure at a range of energies simultaneously. The silicon drift detector avoids the need for a crystal since this would only direct one wavelength into the detector and the whole point of EDXRF is to measure multiple wavelengths. Instead, the detector is normally simply mounted directly close to the sample.
A problem that occurs in current spectrometers is the existence of a background signal in addition to the Bragg reflected signal from the sample that is intended to be measured. This is radiation from a number of sources that is picked up by the detector. The sources of background include scattering from the tube, fluorescent radiation from the sample and contamination in the optical path, the tube, the crystal, and/or the detector.
It is commonly believed that the main part of the background is the tube spectrum scattered by the sample—see for example the text book R. Jenkins, R. W. Gould and D Gedcke, “Quantitative X-Ray Spectrometry”, 1995 R. Dekker, New York, page 408: “The most significant contribution to background is due to the X-ray tube spectrum scattered by the specimen . . . ”.